92 IOWA ACADEMY OF SCIENCES. 



of the time relations. In considering drift sheets which are 

 deployed, the topographic element is of great importance. The 

 pre-Kansan drift is, however, so far as is now known, unexposed 

 except where the Kansan drift has been cut through. The 

 topography of its surface is accordingly almost wholly unknown. 

 The little which we do know, however, is especially significant. 



Physical Gfmracter of Till. — When in studying the indurated 

 rocks one finds above a widespread and characteristic sand- 

 stone, a limestone, a dolomite, or even a sandstone of different 

 character, he suspects at once that he has to deal with a differ- 

 ent formation. To a certain extent the same sort of criteria 

 may be applied in a study of the drift. It has long been recog- 

 nized that marked differences in the character of the bowlders 

 carried betokens differences in the genesis of the drift. Orig- 

 inally this was interpreted as meaning a change in the direc- 

 tion of the ice currents. Recently this has been synthesized 

 and now the phenomena are used to discriminate centers of dis- 

 persion. Aside, however, from the differences in the bowlders, 

 there are certain differences in the physical aspect of the drift 

 itself which come to mean much to the field worker. Such dif- 

 ferences are hard to put into words, and it is not always possi- 

 ble to analyze them and so detect the underlying cause. They 

 cannot always be detected and there are many things which 

 may be deceptive; yet the character of the drift is often very 

 helpful. For example, the yellow clay of the lowan drift is 

 usually more friable than that of the Kansan. The well known 

 "feel" of the loess is another case in point. As a rule the 

 blue clay of the Kansan has the character of a joint clay, 

 breaking with little cubical blocks on drying, etc. 



Cumulative Value of Evidence. — It is a well recognized fact 

 that many isolated bits of evidence have a cumulative value. 

 A fact which standing alone would fail to do more than excite 

 a languid curiosity, when ranged side by side with many simi- 

 lar facts, takes on a deeper significance; while a study of the 

 assemblage of independent evidences will often convince the 

 veriest skeptic. Out of small and individually weak brick, a 

 large and trustworthy wall may be erected. So in the study of 

 the drift sheets. As has been suggested, one class of evidence 

 is rarely found alone; but the whole often unite to make clear 

 a record which could not be deciphered from any one. Even 

 the most intangible of all, the physical aspect of the drift, is 

 often the one first observed, and it serves in no small number 



